Character POSIXct Conversion in R causes wrong timezone values on daylight saving time transition (CEST/CET))
I have a problem converting POSIXct to character and back in POSIXct in R. I run the following code:
time_seq_01 <- seq(as.POSIXct("2012-10-28 02:00:00"), by = 900, length.out = 10)
time_seq_02 <- as.character(time_seq_01)
time_seq_03 <- as.POSIXct(time_seq_02)
or equivalent:
time_seq_01 <- seq(as.POSIXct("2012-10-28 02:00:00"), by = 900, length.out = 10)
time_seq_02 <- format(time_seq_01,usetz = TRUE)
time_seq_03 <- as.POSIXct(time_seq_02)
This are the timestamps in 2012 when the daylight saving transition from Central European Summer Time (CEST) to Central European Time (CET) occurs (Last Sunday in October 02:00 - 03:00).
When I call these elements I get
time_seq_01
[1] "2012-10-28 02:00:00 CEST" "2012-10-28 02:15:00 CEST"
[3] "2012-10-28 02:30:00 CEST" "2012-10-28 02:45:00 CEST"
[5] "2012-10-28 02:00:00 CET" "2012-10-28 02:15:00 CET"
[7] "2012-10-28 02:30:00 CET" "2012-10-28 02:45:00 CET"
[9] "2012-10-28 03:00:00 CET" "2012-10-28 03:15:00 CET"
time_seq_02
[1] "2012-10-28 02:00:00 CEST" "2012-10-28 02:15:00 CEST"
[3] "2012-10-28 02:30:00 CEST" "2012-10-28 02:45:00 CEST"
[5] "2012-10-28 02:00:00 CET" "2012-10-28 02:15:00 CET"
[7] "2012-10-28 02:30:00 CET" "2012-10-28 02:45:00 CET"
[9] "2012-10-28 03:00:00 CET" "2012-10-28 03:15:00 CET"
time_seq_03
[1] "2012-10-28 02:00:00 CEST" "2012-10-28 02:15:00 CEST"
[3] "2012-10-28 02:30:00 CEST" "2012-10-28 02:45:00 CET"
[5] "2012-10-28 02:00:00 CEST" "2012-10-28 02:15:00 CEST"
[7] "2012-10-28 02:30:00 CEST" "2012-10-28 02:45:00 CET"
[9] "2012-10-28 03:00:00 CET" "2012-10-28 03:15:00 CET"
The creation of a POSIXct sequence does work correctly (time_seq_01) the transition to character also (time_seq_02). However the transition back from character back to POSIXct produces wrong timezone (CET/CEST) values (time_seq_03). This can be clearly seen when those elements are sorted:
sort(time_seq_01)
[1] "2012-10-28 02:00:00 CEST" "2012-10-28 02:15:00 CEST"
[3] "2012-10-28 02:30:00 CEST" "2012-10-28 02:45:00 CEST"
[5] "2012-10-28 02:00:00 CET" "2012-10-28 02:15:00 CET"
[7] "2012-10-28 02:30:00 CET" "2012-10-28 02:45:00 CET"
[9] "2012-10-28 03:00:00 CET" "2012-10-28 03:15:00 CET"
sort(time_seq_03)
[1] "2012-10-28 02:00:00 CEST" "2012-10-28 02:00:00 CEST"
[3] "2012-10-28 02:15:00 CEST" "2012-10-28 02:15:00 CEST"
[5] "2012-10-28 02:30:00 CEST" "2012-10-28 02:30:00 CEST"
[7] "2012-10-28 02:45:00 CET" "2012-10-28 02:45:00 CET"
[9] "2012-10-28 03:00:00 CET" "2012-10-28 03:15:00 CET"
This causes a number of problems for instance when merging objects by those timestamps. Is there a way to overcome this problem?
The system I use:
Windows 7 64bit
R version 2.15.1 (2012-06-22)
Platform: x86_64-pc-mingw32/x64 (64-bit)
locale:
[1] LC_COLLATE=German_Austria.1252 LC_CTYPE=German_Austria.1252
[3] LC_MONETARY=German_Austria.1252 LC_NUMERIC=C
[5] LC_TIME=German_Austria.1252
attached base packages:
[1] tools stats graphics grDevices utils datasets methods
[8] base
other attached packages:
[1] pkgtools_0.1-3 roxygen2_2.2.2 digest_0.5.2 rj_1.1.0-4
loaded via a namespace (and not attached):
[1] brew_1.0-6 plyr_1.7.1 rj.gd_1.1.0-1 stringr_0.6.1
Answer
Here's a work around that goes from POSIXct to character back to POSIXct preserving the original daylight savings time status.
Sys.setenv(TZ='Europe/Berlin') # to reproduce OP's example
time_seq_01 <- seq(as.POSIXct("2012-10-28 02:00:00"), by = 900, length.out = 10)
time_seq_02 <- format(time_seq_01,usetz = TRUE)
time_seq_02_lt <- as.POSIXlt(time_seq_02)
time_seq_02_lt$isdst <- as.POSIXlt(time_seq_01)$isdst
time_seq_03 <- as.POSIXct(time_seq_02_lt)
As far as I can tell, R's support for string-to-datetime doesn't include DST flags specified within the strings.